Flat optical metasurfaces are transforming photonics research by enabling new ways to control light in ultrathin, versatile photonic devices. The rise of quasi-bound states in the continuum (qBIC) metasurfaces has enabled tailored high-quality (Q) factor resonances in subwavelength nanostructured thin films, analogous to traditional optical cavities. In this perspective, we explore the emergence of cavity quantum electrodynamics (QED) in optical qBIC metasurfaces, specifically those constructed from van der Waals (vdW) layered materials. Because of their remarkable properties, vdW metasurfaces can support intrinsic optical resonances within the same active material hosting luminescent species, such as excitons or defects, leading to optimal light-matter coupling. This approach of self-hybridizing the cavity-emitter system into a single platform effectively overcomes limitations in on-chip integration of conventional cavities. Combining vdW materials with optically engineered qBIC metasurfaces opens exciting possibilities for exploring nanoscale light-matter interactions. Moreover, the distinctive features of vdW materials, from vertical heterostructures to twist-angle-dependent properties, offer a unique platform bridging the condensed matter physics of 2D materials and engineered nanophotonics. We propose that harnessing strong light-matter coupling in vdW-integrated qBIC metasurfaces will pave the way for next-generation nanoscale polaritonic devices.
The inherently weak nonlinear optical response of bulk materials remains a fundamental limitation in advancing photonic technologies. Nanophotonics addresses this challenge by tailoring the size and morphology of nanostructures to manipulate the optical near field, thus modulating the nonlinear response. Here, we explore a complementary strategy based on engineering the electronic band structure in the mesoscopic regime to enhance optical nonlinearities. Specifically, we demonstrate an increase in second-harmonic generation (SHG) from crystalline silver films as their thickness is reduced down to just a few atomic monolayers. Operating at the boundary between bulk and two-dimensional systems, these ultra-thin films exhibit a pronounced enhancement of SHG with decreasing thickness. This enhancement stems from quantum confinement effects that modify the interaction between electronic states and incident light, which we explain based on quantum-mechanical calculation. Our atomically-thin crystalline silver films provide a new means to overcome the small interaction volumes inherent to nanophotonic platforms, enabling efficient nanoscale nonlinear optics with potential applications in photonics, sensing, and quantum technologies.
Metasurfaces enable diverse applications by controlling light's amplitude, phase, and polarization. Although deep learning-based inverse design has revolutionized metasurface design, current models are limited by fixed operating conditions and lack universality, often requiring retraining for new wavelengths, polarizations, or application scenarios. To address this, we introduce MetasurfaceViT (Metasurface Vision Transformer), a generic AI model for inverse design. Our solution leverages a large dataset of Jones matrices, significantly expanded via physics-informed data augmentation. By pretraining through masking wavelengths and polarization channels, MetasurfaceViT can reconstruct full-wavelength Jones matrices, which are then used by a fine-tuning model for inverse design. This versatility allows one-shot structure design for arbitrary wavelength, polarization, and application requirements. We demonstrate MetasurfaceViT's capabilities in designing multiplexed printings and holograms and broadband achromatic metalenses. Prediction accuracy exceeds 99% for physically realistic designs, showcasing a significant step toward a universal optical inverse design paradigm.
Optical metasurfaces are widely studied due to their unprecedented wavefront modulation capabilities for multiple polarization channels. Current studies predominantly focus on complete polarization conversion. Recent progress indicates that the phases of quadruplex polarization channels can be independently modulated under incomplete polarization conversion conditions. However, these four-channel phase modulation operations are limited to circular or linear polarization states and neglect amplitude modulation. Here, a strategy is proposed to achieve four-channel phase modulation and flexible energy distribution of arbitrary orthogonal polarization states under incomplete polarization conversion conditions. Wavefront modulations for quadruplex channels of arbitrary orthogonal polarization states (circular, linear, elliptical, and first-order cylindrically vectorial), such as orbital angular momentum manipulation, Bessel beam generation, deflection, and holography, are numerically demonstrated based on this strategy. Furthermore, the energy distribution of the quadruplex polarization channels is achieved by varying the polarization conversion efficiency. These operations are implemented through all-dielectric free-standing bilayer metasurfaces. The proposed design strategy extends the application of metasurfaces in multichannel optical field modulation.
Broadband directional thermal emitters have attracted significant attention due to their potential applications in infrared camouflage and radiative cooling. However, existing broadband directional thermal emission (BDTE) multilayer structures rely heavily on the Berreman modes of epsilon-near-zero (ENZ) materials, usually requiring a substantial number of stacked ENZ thin films for broader spectral coverage. Moreover, the lack of optimized thicknesses fails to achieve the optimal figure of merit (FOM) of BDTE. Here, we have realized a high-FOM BDTE structure with a reduced number of ENZ layers based on Bayesian optimization. By coupling epsilon-near-pole (ENP) resonance with the Brewster effect of the dielectric spacer, we extend the BDTE bandwidth by 2 μm (from 7.9-12 to 7.9-14 μm). The optimized structure shows unprecedented performance, achieving an average directional emissivity of 0.94 and an FOM of 8.087, which are also validated by experimental measurements. Notably, by integrating our emitter with low-emissivity covers, we develop a series of patterned devices for infrared information encryption and deception applications, which exhibit angle-dependent distinct, even contradictory, infrared information. This work not only provides theoretical guidance for the design and optimization of BDTE structures but also paves the way for their applications in infrared information technologies.
Dark states of photoluminescence (PL) intermittency in colloidal quantum dots (QDs) interrupt PL emission and significantly reduce emission intensity, severely hindering QD applications. However, the origin of dark states remains ambiguous due to their extremely low intensity, which impedes the development of effective suppression strategies. In this study, we use plasmonic gold nanoparticles to significantly increase the radiative rate of excitons, and thereby enhancing the dark-state PL intensity. Calculations of radiative rate scaling based on the dark-state PL intensity and lifetime reveal that the dark states originate from band-edge carrier trapping by collectively activated nonradiative multiple recombination centers (MRCs). Transition states that accompany the dark states are frequently observed in PL trajectories, revealing the presence of a positive feedback mechanism for the activation and deactivation of nonradiative MRCs induced by the phonon kick effect. We perform a Monte Carlo simulation to model the dark and transition states and quantify the nonradiative rates involved. Understanding the origin of dark states can contribute to their suppression, optimization of synthesis, and improvement of performance in QD-based applications.
Exciton dissociation in semiconducting nanostructures is crucial for optoelectronic applications, especially when free-carrier generation is required. Despite considerable research, the question of whether and how such generation occurs in strongly excitonic systems remains elusive. Here, we use one-dimensional precision graphene nanoribbons (GNRs) as a model system to investigate exciton dissociation. We systematically explore the interplay between ribbon length (l), excitation energy, and band dispersion in various precision GNRs. Ultrafast Terahertz conductivity measurements reveal that hot exciton dissociation dominates carrier generation, with ribbon length significantly influencing free carrier lifetimes. We identify a critical Bjerrum length (RB) of approximately 20 nm that determines whether photoexcited hot carriers in GNRs can dissociate before forming tightly bound excitons. For shorter ribbons (l < 2RB), rapid ~ps exciton formation prevails. Furthermore, the charge-carrier band dispersion in GNRs plays a critical role in determining dissociation efficiency. Long GNRs with strongly dispersed bands, and consequently low effective carrier masses, exhibit higher mobilities that promote efficient hot-exciton dissociation. These results advance fundamental understanding of dimensionality, energetics, and electronic structure in excitonic materials, providing design principles for optoelectronic devices based on excitonic materials.
Vortex beams, known for carrying orbital angular momentum (OAM), demonstrate significant potential across LiDAR, laser communications, high-precision metrology, imaging, and quantum information. Generating vortex beam on demand is the basis and crucial for above implementations. Recently, solid-state vortex laser in eye-safe band have attracted considerable attention and been regarded as one of ideal sources for vortex beam generation, because of its advantages in high mode purity, mode selectivity, high output power, and locating in atmosphere window. In this perspective, we summarize the schemes of solid-state vortex lasers in eye-safe band, survey their potential in multi-functional LiDAR systems, and discuss prospects for future development.
Skyrmionic patterns of optical fields have recently emerged across diverse photonic platforms. Here, we show that such textures also arise in the polarization eigenstates of light propagation through flat dielectric devices with an engineered, space-dependent optic axis orientation. We focus on two-dimensional periodic structures, where propagation through multiple devices maps onto quantum dynamics on a synthetic optical lattice. Adopting the condensed-matter framework, a spatial period defines an effective Brillouin zone, and polarization eigenstates can be grouped in two bands, with the role of energy played by the opposite phase delay. When such eigenstates exhibit skyrmionic textures, the corresponding lattice model shows the topology of a Chern insulator. We validate these concepts in a system of three tunable liquid-crystal metasurfaces. Using machine learning, polarization eigenmodes are reconstructed over one spatial period. We identify configurations of the devices' parameters that lead to topologically nontrivial bands, where we directly observe skyrmionic eigenpolarization textures. We also extract local observables of lattice models, such as the Berry curvature and the quantum metric. We finally report a numerical simulation of an all-optical quantum Hall effect emerging when light propagates through a sequence of such devices, arranged to mimic the effect of an external force on the lattice.
Morphological transformations are playing a key role in visual information processing with diverse applications ranging from bioimaging to video surveillance and environmental monitoring. However, these operations are becoming increasingly computationally intensive, requiring substantial memory and processing power as the size of image datasets expands. This paper describes a fast, highly parallel approach to perform morphological transformations by diffractive computing. These all-optical processors consist of successive diffractive surfaces designed to perform dilation and erosion operations by learning the relations between input and transformed images via a deep learning-based optimization process. Unlike existing digital methods, our free-space diffractive devices implement these transformations in a computer-free manner by directly processing the optical wavefront. The cascaded diffractive architecture further enables image denoising and flexible tuning of the extent and directionality of erosion/dilation through the same training process by adjusting target image datasets, realizing the synthesis of diverse transformation kernels on demand. We also demonstrate that the optical process is scalable and can process large volumes of visual information in a highly parallel manner. Experimentally, we realize such a diffractive network in a reflection configuration using a phase-only spatial light modulator (SLM) and perform morphological transformations on both amplitude- and phase-encoded images.
Photon echo (PE) techniques offer a promising approach to optical quantum memory, yet their implementation in conventional platforms, such as rare-earth-ion-doped crystals, is hindered by limited bandwidths. Semiconductor quantum dot (QD) ensembles, featuring THz-scale inhomogeneous broadening and sub-picosecond dynamics, provide an attractive alternative for ultrafast applications. However, achieving coherent control across such broad spectral ranges remains challenging due to detuning and spatial field inhomogeneities, which reduce PE efficiency. In this work, we demonstrate that chirped rephasing pulses satisfying adiabatic conditions enable robust adiabatic rapid passage (ARP) across an inhomogeneously broadened InAs QD ensemble. This approach achieves uniform population inversion and broadband rephasing, overcoming the limitations of transform-limited excitation. Experimentally, we observe a 3.2-fold enhancement of the PE signal in dense, self-assembled InAs QDs operating at telecom wavelengths. Numerical simulations based on a two-level model reproduce the experimentally observed ARP-induced enhancement, validating the underlying physical mechanism. These results establish ARP as an effective and scalable method for coherent control in THz-broadened QD ensembles, opening a pathway toward ultrafast and broadband optical quantum memory and communication in the telecom band.
Bound states in the continuum (BICs) are waves exhibiting theoretically infinite quality factors, offering a powerful mechanism for extreme light confinement in photonic structures. Although breaking vertical structural symmetry in BICs-supporting systems can induce asymmetric radiation, the radiated power typically remains partitioned between opposing half-spaces. Furthermore, achieving arbitrary control over the amplitude ratio and phase difference of these counter-propagating beams presents a significant challenge, thereby limiting sophisticated beam manipulation within a single half-space. In this work, we delve into BICs within the superwavelength regime, where photonic structures inherently support multiple diffraction orders. We systematically investigate the far-field polarization states and associated topological properties of these individual diffraction channels. Critically, by engineering a configuration that supports two co-propagating diffraction orders directed into the same half-space, we demonstrate comprehensive and continuous control over the resulting unidirectional guided resonances (UGRs). Full tunability of both the directionality (spanning from -1 to 1) and the relative phase difference (spanning from -π to π) between these two co-propagating beams is achieved. This versatile manipulation of multiple beams radiating concertedly into a specific direction opens new avenues for various advanced applications.
Surface phonon polaritons (SPhPs) enable nanoscale manipulation of mid-infrared light via deeply subwavelength topological vector textures, such as skyrmions. Achieving dynamic, real-time control over these topological features remains challenging. Here, we theoretically propose and numerically demonstrate an actively tunable platform on a silicon carbide membrane that creates lattices of diverse topological textures, including skyrmions, merons, and skyrmion bags. By exploiting the sublinear SPhP dispersion, we dynamically adjust the excitation wavelength to tune the topological character of these lattices. This enables tunability between bubble-type and Néel-type configurations, controlling field confinement and topology for topological textures in the electric field and the spin angular momentum. Furthermore, we identify a novel singularity-type meron arising from the interplay of electric and magnetic spin components. This texture exhibits topological charge conservation in moiré superlattices and a spatially confined spin reversal with tunable lateral sizes as low as λ SPhP / 29 and skyrmion number density confinements as low as λ SPhP / 64 . These findings provide a versatile framework for on-chip, reconfigurable topological photonic devices with potential applications in high-resolution imaging and precision metrology in the mid-infrared. The results can be readily extended to other topological systems, where similar dispersion relations hold.
Analytical multilayers designed under quarter-wave conditions, such as antireflective coatings and distributed Bragg reflectors, generally perform effectively within narrow spectral bands but often face challenges in meeting multispectral demands. In contrast, machine learning (ML)-driven inverse design enables exploration of vast parameter spaces to realize tailored spectral responses across multiple bands. However, whether ML-optimized multilayers can outperform analytical designs under identical material and thickness constraints often remains an open question. Here, we experimentally validate the superiority of ML-driven design through a metal/dielectric multilayer cooling-window coating that simultaneously requires high average visible transmittance (AVT) and high average near-infrared reflectance (ANR). By integrating a factorization machine with simulated annealing, we discovered optimized aperiodic ZnS/Ag multilayers and benchmarked them against periodic hyperbolic metamaterial (HMM) counterparts. Under a 156 nm thickness constraint (equivalent to two ZnS/Ag pairs in a HMM), the ML design achieved 0.57 AVT and 0.98 ANR, surpassing the HMM reference (0.49 AVT, 0.83 ANR). With an extended thickness of 300 nm, the ML-optimized coating further improved to 0.79 AVT by suppressing Fabry-Perot resonances while maintaining high ANR (0.97). Furthermore, the ML-driven multilayers exhibited tunable transmitted colors spanning the full visible gamut, whereas the HMM counterparts were restricted to specific hues. Both ML and HMM designs were fabricated on glass, and measured spectra confirmed the superior optical and thermal performance of the ML approach. These findings establish ML-driven inverse design as a powerful route to ultrathin, manufacturable, and color-tunable cooling-window coatings that can contribute to urban energy savings.
Microsphere-lens-assisted optical nanoscopy has emerged as a powerful approach for surpassing the diffraction limit of conventional optical microscopy. Here, we present a comprehensive investigation of high-refractive-index fluorotellurite (TeO2-BaF2-Y2O3, TBY) glass microspheres fabricated by a high-temperature floating-zone melting technique. The microspheres exhibit excellent sphericity, ultra-smooth surfaces, diameters from 10 to 200 μm, a refractive index of ∼1.9, and up to 85% visible transmittance. Ray-tracing and full-wave electromagnetic simulations qualitatively and quantitatively characterize their near-field focusing and efficient evanescent-to-propagating wave conversion. When fully embedded in a PDMS matrix, TBY microspheres enabled super-resolution imaging of anodic aluminum oxide and other nanoscale samples, resolving features down to 50 nm and attaining a maximum magnification of ∼4.34× on 100 nm grating structures. We show that image-plane selection and precise axial alignment critically influence image clarity, contrast, and magnification, and we systematically investigate these trade-offs across sphere diameters. An ultramicroscopic objective (UO) module integrating a plano-convex lens with an embedded microsphere was developed to provide micrometer-precise positioning, reusability, and straightforward compatibility with commercial microscopes. The high near-infrared transmittance, low dispersion, and thermal stability of fluorotellurite glass indicate promising applications in deep-tissue near-infrared super-resolution, multi-band spectroscopic nanoscopy, and laser micro-machining.
A vortex beam at eye-safe band is emitted from a solid-state laser resonator. Such beam carries orbital angular momentum and is employed to sense the spinning cyclone via the rotational Doppler effect. As an ideal class of vortex sources, solid-state vortex lasers at eye-safe band have attracted considerable attention due to their potentials in multi-functional LiDAR. In the Perspective Article (DOI: 10.1002/nap2.70006), Shiyao Fu and co-workers summarize the schemes of eye-safe vortex lasers and discuss their prospects for future development.
Manipulating propagating waves (PWs) and surface waves (SWs) in desired manners is important in photonics, but controlling these two electromagnetic modes usually requires separate devices, which is unfavorable for integration optics applications. Recently, although metasurfaces capable of controlling both PWs and SWs have been proposed, they typically rely on dynamically varying the helicities of incident circularly polarized (CP) light, causing complexities in practical applications. In this work, we propose an alternative scheme for designing metasurfaces encoded with both resonance and geometric phases that can simultaneously control PWs and SWs through the co- and cross-polarized output channels under the excitation of a CP wave with a particular helicity. We experimentally prove this concept by realizing two microwave metadevices that can convert normally incident beams with left circular polarization (LCP) into PWs and SWs with predetermined wavefronts. Additionally, we numerically demonstrate how to design metadevices with predetermined energy distributions within these two functional output channels. Our work paves the road to tailor both far- and near-field electromagnetic waves using a single ultra-compact platform, which can find many applications in integrated optics.
Optical neural networks leverage the inherent parallelism of light to multiplex across various degrees of freedom including wavelength, polarization, and modes. Among these, orbital angular momentum (OAM), possessing a theoretically infinite number of orthogonal mode dimensions, holds significant potential for constructing optical neural networks. However, OAM conversion and multiplexing on integrated photonic chips remain challenging. Here, we present an on-chip OAM mode converter and multiplexer device based on inverse design. The OAM mode converter achieves maximum up-conversion efficiency of 88.68% ( OAM - 1 → - 2 ), maximum down-conversion efficiency of 88.04% ( OAM - 3 → - 1 ), and maximum modulation depth of 4.07 dB ( OAM + 1 → + 3 ). Besides, the OAM ± 1 , ± 2 multiplexer achieves maximum conversion efficiency of 98.29% and maximum modulation depth of 20.69 dB. Subsequently, we demonstrate an OAM-encoded hybrid optical convolutional neural network built using this device, achieving 98.0% accuracy on MNIST handwritten digit recognition and 86.1% accuracy on Fashion-MNIST classification. This device provides a novel approach for on-chip OAM conversion and multiplexing while also enabling on-chip optical convolution operations by using OAM mode. This work offers a practical pathway for integrating OAM with on-chip optical neural networks.
The rapid progress of microwave imaging technology has made conventional camouflage materials with fixed absorption performance ineffective. As the imaging band expands, camouflage materials capable of broadband operation, especially in the S and the C band, dynamic modulation are required to hide targets in complex environments. Here, we propose a dynamically modulated camouflage metasurface employing deep-subwavelength slots to enhance multiband modulation capability. By varying the vertical displacement of the structure, reflectivity can be modulated from below -10 dB to near 0 dB over 2.7-19.1 GHz (150.4% relative bandwidth), while maintaining insensitivity to the incidence angle and polarization. An equivalent interface-impedance model is established to reveal the mechanism of slot-enhanced low-frequency resonance. The laser processing parameters are optimized to reduce the slot width to 25 μm (λ max/4440), enabling broadband dynamic modulation, as experimentally verified. Benefiting from its broadband dynamic modulation performance extending to the S and the C band, the proposed camouflage metasurface demonstrates potential for countering emerging microwave imaging technologies.